Coaxial filter



MilCh 10, 1959 DEVQT- COAXIAL FILTER Filed Nov. 1, 1954 IN V EN TOR.

United States Patent 'COAXIAL FILTER Andre J. Devot, Canton, Mass., assignor to Tobe Deutschmann Corporation, Norwood, Mass., a corporation of Massachusetts Application November 1, 1954, Serial No. 466,175

2 Claims. (Cl. 333-73) The present invention relates to a coaxial filter for filtering out electro magnetic waves over a substantially wide range of frequencies, particularly in ranges from one thousand mcs., up to about twenty thousand mcs., in which the attenuation is as much as one hundred db.

Filters of the present type are not only useful in testing circuits, but are also useful in ordinary low frequency circuits where interference from the higher frequencies is desired to be eliminated.

The filter of the present invention is an improvement on the filters shown and described in my U. S. Patent 2,700,136, issued January 18, 1955, wherein abrupt changing sections establishing reflections of undesired frequencies, which it is desired to filter out were used.

The invention, in its simplest construction may be formed of series of repeated sections in which each side of the line may be formed with a central through conductor surrounded by concentric tubular elements providing the necessary impedance changes for effecting the filtering action.

The invention will be more fully described in the specification below when taken in connection with the drawing attached hereto, showing a broken longitudinal through section through the coaxial cable filter unit. The coaxial filter used in the present invention is preferably used in each side of the line and may be used when running the cable through a screen or a shielded housing for eliminating a magnetic field. As indicated in the figure, the filter comprises an outer tubular element 1, which abuts against end insulating washers 2, 2 and a central conductive rod 3, to which is threaded end nuts 4 and 5, on threaded enlarged end sections 6 and 7 respectively, of the conducting rod 3. The end nuts 4 and 5, have inner surfaces which press the nuts against the insulating washers 2, which may be of melamine or other insulating plastic material. The conducting rod 3, has fitted to it at each end a threaded collar or stud 8, which has an internal flange 9, the stud 8 being screwed into the inner end of the internally threaded nuts 4, so as to secure a capacitor cup 10, which is formed with a cylindrical section spaced parallel to the tube 1, and a flanged section 11, which is clamped tightly between the flange 9, of the stud 8, and the insulating washer 2, at the end of the cylinder. The cup 10, forms a capacitance with the outer cylinder 1, in such a way as to reduce the inductance of this capacitance to an absolute minimum. Lying against the face of the flange 9 is a silicone tube section 12. A ferrite core section 13 abuts against the section 12 and another silicone tubing section 14 is disposed on the other side of the ferrite core section. These elements 12, 13 and 14 all surround the inner conductor 3, and the ferrite core 13 provides an increased inductance element for the lower "ice frequencies passing through the inner conductor 3. This ferrite core sleeve surrounding the conductor 3 may be the usual ferrite alloy core and preferably is of the type which is pressed together from ferrite powder mixed with a suitable binder, which is used in the manufacture of cores under high temperature conditions. The ferrite core lowers the cutoff frequency and thereby increases the stop band. This is accomplished by an increase in the inductance of the inner conductor. At the end of the silicone tube or sleeve section 14, is a shield disc or washer 15.

Centrally located in the second section of the coaxial filter is a ferrite core 20, which surrounds the conductor 3, on either side of which are the silicone sleeves or tubes 21 and 22. The third section of the filter shown also includes a center ferrite core 24, on either side of which is a silicone tube section 25 and 26.

These latter two core sections are surrounded by twin capacitor cups 27 and 28, which have their bases 29 and 30 back to back with the open ends of the cups facing in opposite directions at the end of the sleeve element 31 and 32 of the cups. The section 34 of the cup is similar to the section 33 but is inverted. The other sections of the filter (not visible behind the unbroken portion of the tubular element 1) would be like the first two sections of the filter previously described.

In the present invention, the ferrite core is substantially of a very high resistance material, but made of a sintered ferrite compound providing more noticeable magnetic properties for frequencies from the low range of several hundred cycles per second to several thousand megacycles per second. Beyond the range of the measuring instruments or devices with which the present filter is used the attenuation is greater than db from low frequencies up to about 20,000 megacycles per second. This structure is primarily adapted for use with parallel wire systems although it may readily be adapted for coaxial systems. Connections are made in the line being used in a manner similar to that set forth in my issued United States patent mentioned above.

Having now described my invention, I claim:

1. An electric line filter comprising a central coaxial conductor, an outer coaxial conductor twin capacitor cup sections having a section coaxial with the central conductor and a section at right angles thereto connecting said central conductor with the cups back to back with one another, the central coaxial conductor having a plurality of ferrite sleeves, one for each cup section centrally located in each cup section.

2. An electric line filter comprising a central coaxial conductor, an outer coaxial conductor, an end section having a ferrite sleeve in the center of the section and a capacitor cup with the base of the cup at right angles to the central coaxial conductor surrounding said ferrite sleeve, an end insulating washer, means securing the base of the cup to the end insulating washer, an outer conducfor tube surrounding but spaced from the sides of the capacitor cup and end securing means attached to the central coaxial conductor for securing the elements of the section together.

References Cited in the file of this patent UNITED STATES PATENTS 2,228,797 Wassermann Ian. '14, 1941 2,548,881 Ferrill Apr. 17, 1951 2,700,136 Devot Jan. 18, 1955 2,782,381 Dyke Feb. 19, 1957 

